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LT1500

Adaptive-Frequency Current Mode Switching Regulators

Linear Technology 

LT1500/LT1501

APPLICATIONS INFORMATION

with an external divider no switch exists and the divider

current remains. There may be additional current drawn

by the adjustable LT1500 in shutdown if the divider

voltage at the feedback node exceeds 0.6V. See Typical

Performance Characteristics.

Loop Stability

The LT1501 is internally compensated since the device

has no spare pin for a compensation point. The LT1500

brings out the VC pin to which an external series RC

network is connected. This provides roll-off for the error

amplifier, ensuring overall loop stability. Typical values

when using tantalum output capacitors are 1000pF and

100kΩ.

Transient response of Figure 3’s circuit with a 30mA to

100mA load step is detailed in Figure 4. The maximum

output disturbance is approximately 20mV. The “splitting”

of the VOUT trace when load current increases to 100mA is

due to ESR of COUT. COUT can be replaced by a ceramic

unit, which has lower ESR, size and cost. Figure 5 shows

transient response to the same 30mA to 100mA load step,

with COUT = 15µF ceramic, CC = 2200pF and RC = 10k. The

maximum output disturbance in this case is 100mV.

Low-Battery Detector

The low-battery detector is a combined reference and

comparator. It has a threshold of 1.24V with a typical input

bias current of 20nA. In a typical application a resistor

divider is connected across the battery input voltage with

the center tap tied to Low Battery Input (LBI), see Figure

6. The suggested parallel resistance of the divider is 150k

VCOMP

500mV/DIV

VOUT

20mV/DIV

AC COUPLED

ILOAD 100mA

30mA

IL

500mA/DIV

500µs/DIV

Figure 4. Transient Response of LT1500 with RC = 100k,

CC = 1000pF and COUT = 220µF. VOUT Disturbance is 20mV

VCOMP

500mV/DIV

VOUT

50mV/DIV

33µH MBR0520L

AC COUPLED

VIN

CTX33-1

2V

VOUT

5V

ILOAD 100mA

30mA

VIN

SHDN

ISENSE

SW

1M

100pF

IL

500mA/DIV

FB

LT1500

VC

GND PGND

332k

RC

100k

CC

1000pF

*TANTALUM = AVX TPS SERIES

CERAMIC = TOKIN 1E156ZY5U

Figure 3. LT1500 2V to 5V Converter

COUT*

220µF

200µs/DIV

Figure 5. Transient Response of LT1500 with RC = 10k, CC =

2200pF and COUT =15µF Ceramic. VOUT Disturbance is 100mV

LT1500/01 • F03

R3

301k

VIN

1%

LBI

LT1500

LT1501

LBO

R4

274k

GND

1%

R5

10M

VCC

470k

PULL-UP RESISTOR

SHOULD BE AT LEAST

FIVE TIMES SMALLER THAN

R5 TO ENSURE LBO

HIGH STATE

LT1500/01 • F06

Figure 6. Low Battery Detection

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